JP6243225B2 - Thermograph test method and test apparatus for carrying out this test method - Google Patents

Description

  The present invention relates to a thermographic test method for local resolution detection and identification of defects near the surface of a test object, and a test apparatus suitable for carrying out the test method.

  Semiconductive semiconductive products, such as metal billets, steel bars, bars, tubes, or wires, often serve as starting materials for high-end end products and often have to meet very high quality requirements. Testing for defects in the material, particularly near-surface defects such as cracks, voids, or other inhomogeneities in the material, forms an important part of quality control of these products. During this test, it is generally endeavored to test the surface of the material as completely as possible using a high local resolution and this test is carried out as early as possible in the production chain and the results of the test Depending on the type of defect found, determine whether the defect is not important for further processing, whether it can be repaired at least by rework such as grinding, or the material should be discarded .

  Apart from magnetic methods often used for such tests, such as eddy current technology or stray magnetic flux technology, in recent years thermographic test methods have also been used for local resolution detection and identification of defects near the surface under test. Has been used for.

  In known thermographic test methods, a conductive test object, such as billet steel, after rolling passes an induction coil under high frequency alternating current to induce a current near the surface of the test object. Due to the skin effect that depends on the excitation frequency, the current density in the vicinity of the surface of the specimen is greater than in the interior of the test object. Microstructural faults, such as cracks, in the induced current cross-section, act as electrical resistance, causing the current to redirect to find the path of least (electrical) resistance in the specimen material. . This results in a higher current density and thus a greater power loss in the current “constriction” in the region of the defect. Although locally limited, the affected region directly adjacent to the microstructural disturbance exhibits a higher temperature compared to the unaffected peripheral region, so that the power generated in the region of the microstructural failure The loss is recognized by the heat generated. Using a thermal sensing camera or other suitable recording device that is sensitive to thermal radiation, the presence of defects near the surface is locally resolved based on local temperature values in the surface area recorded by the recording device. It can be detected by a method of imaging. A visual representation of the recorded surface area is also provided, and the anomalies identified by the thermograph are automatically evaluated by a downstream evaluation system.

  Patent Document 1 discloses a thermograph test method and a test apparatus assembled to perform the test method. The test apparatus includes an induction coil for heating a surface area of a metal under test flowing through an induction coil, such as billet steel, and one or more infrared rays for measuring the temperature profile of the billet steel passing therethrough. And a camera. The measurement results are used to activate the color marking system to mark the defects found. To evaluate thermographic images (thermal images) recorded by an infrared camera, this description provides evaluation software that analyzes the thermal images, identifies temperature differences that exceed a predetermined threshold, and reports these as defects. The The evaluation software evaluates defects for both the length and size of temperature differences that exceed a threshold. The evaluation software can remove defects with a length less than the minimum defect length from the defect list, so that these defects are no longer found as defects. However, defects are below the minimum defect length, but the size of the temperature difference exceeds the threshold, and even if it exceeds the maximum size of the temperature difference, such a defect is still reported as a defect. Thus, the defect is identified according to the defect length and the temperature difference with respect to the peripheral region.

  An increase in temperature profile greater than 2K relative to the peripheral region is generally considered a defect, but the threshold temperature can also be selected to be lower. A temperature difference of 5K or more relative to the surrounding area is clearly identified as a defect.

  In practice, the temperature profile to be evaluated is typically overlaid with an appreciable amplitude interference signal. Potential sources of interference include environments that are generally inevitable in actual test operations, such as local variations in the extent of radiation on the surface under test, reflections from surrounding areas, and foreign objects on the surface of the specimen. . For example, the edge on the positive profile often has a higher temperature than the surrounding area, so that the display error may occur due to the geometric shape of the test piece. Typically, the temperature difference caused by crack-like defects compared to the peripheral surface is on the order of 1K to 10K. It has also been found that the amplitude of interference can be on the order of this magnitude. Thus, even if all possible measures are taken to reduce the amplitude of interference, it cannot be ruled out that the interference is misclassified as a microstructural flaw or defect.

German Patent No. 10 2007 055 210 A1

  The problem addressed by the present invention is to provide a thermograph test method and a thermograph test apparatus suitable for performing the method, which provides improved interference suppression in the evaluation of thermograph signals compared to the prior art. To do. In particular, it is intended to improve selectivity in distinguishing between actual defects and pseudo defects due to other interference. Preferably, it is intended to test the entire surface of an elongate object of conductive material with increased defect detection and identification reliability.

  In order to solve this and other problems, the present invention was assembled to perform a thermographic test method having the features of claim 1 and the method having the features of claim 10. A thermographic test device is provided. Advantageous developments are identified in the dependent claims. The expression of all claims refers to what is described.

  In the test method, the part to be tested to be tested is exposed to the action of the heating device. Hereinafter, this is also referred to as “heating up” for short. In this case, the heating energy is induced such that a thermal imbalance occurs between the defect area affected by the defect, i.e., the location of the flaw and the material of the specimen without the defect. The scratched area, that is, the defective area in this case, includes an actual flaw such as a crack and a directly adjacent peripheral area. A defect free peripheral area can hold the temperature under the action of the heating device, i.e. it is not heated or heated more intensely than the scratched area.

  In the case of conductive test objects such as metal billets, steel bars, bars, wires, etc., induction methods can be used, for example for the heating process. The input of thermal energy to the defect area to be tested can also be performed using ultrasonic waves.

  In the heat propagation phase, a series of two or more thermographic images are recorded at time intervals from one another. The heat propagation phase begins when there is a heat flow from the locally heated defect area to the surrounding area. The heat propagation phase continues to the cooling phase following the heating process and often coincides with the cooling phase. However, there are often no strict boundaries between the heating and cooling phases. Since thermal energy can already propagate during the heating process, the initiation of the heat propagation phase can still overlap in time with the local heating phase.

  Each of the thermograph images in this case represents a local temperature distribution in the surface area of the test object recorded by the thermograph image at different times during heat propagation. When the recording device provided for recording the thermographic image, such as a heat sensitive camera, and the test object are in a stationary state, the surface area of the test object recorded at different times may be the same. When there is relative motion between the test object and the recording device, the surface areas are spatially offset with respect to each other.

  Positionally properly assigned temperature profiles are determined from a series of thermographic images, where each of the positionally properly assigned temperature profiles is assigned to the same measurement area of the surface to be tested. The term “measurement area” refers here to an area extending in one or two dimensions having a fixed position in the coordinate system under test. Many measurement locations exist within the measurement area.

  The term “temperature profile” refers to a local resolution profile in which different locations or locations within the temperature profile are each assigned a value of a measurement variable that represents the temperature at each location. The temperature profile can be understood as a position function that explains the dependence of the temperature value on the position in the temperature profile. The temperature profile can be for a somewhat narrow, almost linear region, such as a spectral line change diagram. The temperature profile may also relate to a 2D profile or region profile, and then the local distribution of temperature values in a piece of a given shape and size is described by the temperature profile. Measurement variables assigned to different positions in the temperature profile can be referred to as “temperature values”. This generally does not require direct measurement of the temperature, but it does require, for example, measuring the intensity or amplitude of the thermal radiation emitted by each location, which can be obtained by means of conventional means in thermography. Can be converted to a local temperature.

  In this way, multiple temperature profiles (at least two) are determined, representing local changes in temperature within the same measurement region at different times during the cooling process. Then, the change over time in the temperature value is quantitatively determined from the temperature profile for many measurement positions of the measurement area recorded by the temperature profile, and the local temperature value over time is determined by the multiple measurement positions in the measurement area. To get about. The change over time is then evaluated based on at least one evaluation criterion suitable for characterizing the heat flow in the measurement region.

  In this method, not only the temperature profile related to the local change in temperature represented by the temperature profile, but also its change over time is analyzed. A sequence of temperature profiles or a series of temperature profiles is acquired for a predetermined measurement area and a predetermined time range on the surface. An essential aspect of the method is that it includes the dynamic behavior of heat flow, ie the development of temperature profiles over time, and its evaluation or interpretation.

  Therefore, according to another form, the use of a variant of the local resolution heat flow thermography for the detection and identification of defects near the surface of the preferred test object is proposed, and the time distribution of the local distribution of temperature found on the surface of the specimen is proposed. Expression is identified and evaluated. Among other things, this involves quantitatively recording and evaluating the lateral heat flow.

  Compared to the prior art, the method allows an improved ability to distinguish between temperature effects due to defects and effects that are not caused by heat flow, so classification of defects such as cracks or microstructural defects, for example. Is much more certain. Furthermore, not only is the temperature signal amplitude or intensity in the profile deterministic, but also how the temperature signal behaves dynamically in the time axis, so even if the signal amplitude is small, the thermograph information The ability to evaluate is improved. This significantly improves interference suppression even when the interference amplitude (not attributed to the defect being sought) is greater than the useful signal amplitude. As used herein, beneficial signal amplitude refers to signal amplitude caused by microstructural disturbances.

  With this test method, the spatio-temporal heat propagation after a sudden locally limited inflow of heat can be recorded and evaluated quantitatively. In short, spatio-temporal heat propagation occurs so that heat collected in the area of potential defects flows over time into the surrounding, cooler area of the material under test. This inflow is clarified by the temperature distribution on the lateral surface as long as the amplitude of the temperature profile during excitation decreases with time, but the temperature rises significantly in the immediate vicinity of the excitation position. Therefore, under these conditions, the shape of the temperature profile changes characteristically with time. On the other hand, the effects of interference that occur most frequently, such as surface reflections, are either not changing at all over time or only slightly with respect to their local properties and / or clearly from typical heat flow behavior. Deviates over time (eg, flashes with short reflections). Thus, the effects of such interference can be clearly distinguished from actual defects based on their typical spatiotemporal behavior. In fact, some interference effects are manifested in the temperature profile by dynamic spatiotemporal behavior, which is the spatiotemporal that occurs in the peripheral region of defects in the thermally conductive material that is not affected by interference. In general, it is distinctly different from mechanical heat propagation. Thus, the evaluation of analyzing the spatiotemporal behavior of the temperature profile from the aspect of the principle of heat propagation or heat diffusion in solids provides improved selectivity and interference suppression compared to conventional methods.

  Thus, the assessment is also described as including an assessment of the comparison of the recorded thermograph data with the signature, which is a solid that attempts to re-establish thermal equilibrium, especially after local concentration of heat. Is a description of spatio-temporal heat propagation.

  In the preliminary evaluation step, it is preferable to automatically analyze the temperature profile as to whether there are obvious defects in the temperature profile that may be due to defects but not necessarily. In identifying anomalies that are likely to be defects, it is preferable to look for a local maximum of temperature values in the temperature profile. Here, the local maximum corresponds to a location in the temperature profile at a temperature clearly higher than the temperature at the profile position in the region directly surrounding the local maximum. For example, in a crack test, the identification step is intended to find a substantially narrow hot location in other cooler peripheral regions. In this identification step, for example, over a short distance from one side of the peripheral region to the other side of the peripheral region, so to speak, the local maximum is assumed to be a temperature that rises or falls stepwise. Appropriate image processing filter routines can be used to distinguish from position. Generally, for this purpose, two or more filter routines that operate on different criteria are used to identify image locations (pixels or groups of pixels) that are clearly due to local maximum temperatures. Can do.

  The evaluation can then concentrate on the area where the local temperature maximum is found. In a modification of the method, the change over time in the amplitude of the temperature value in the region of the local maximum of the temperature value of the temperature profile is evaluated as an evaluation criterion. This can be used, for example, to determine the cooling rate in the local maximum region and its vicinity. It turns out that the cooling rate in the surrounding area, which is not affected by the other, and where there are microstructural defects such as cracks, can be well explained by the principle of thermal diffusion and as a result can be used as a reliable evaluation criterion. ing. Thus, cracks and other defects are often distinguishable from faults not due to defects based solely on typical cooling rates.

  Alternatively or additionally, the evaluation can determine a heat volume concentration value in a local maximum region of the temperature value in the temperature profile, and determine the change in the heat volume concentration value over time. Can be evaluated. The hot volume concentration value is a measure of how the local maximum heat is related compared to the directly surrounding area. If this hot volume concentration value decreases with time, heat flows into the surrounding area, for example, as is typical in the surrounding area of the crack. On the other hand, if the local maximum is not due to microstructure failure or cracking, the heat concentration value may behave significantly differently and may even continue to rise initially after the heating process is finished. . This is then an indication that the local temperature maximum is not due to cracks or the like.

  In a preferred embodiment, at least three recorded in succession to each other in time so that a suitable time function can be determined with sufficient accuracy for the evaluation of the change over time as a calculated characteristic variable. Evaluate the temperature profile together to obtain the appropriate number of interpolation locations. In general, 4 to 10 temperature profiles are evaluated together, and there are a sufficient number of interpolated positions within the time range to reliably distinguish between defects and artifacts.

  Alternately or in addition to determining and evaluating feature variables from a time function, changing temperature values in a temperature profile over time based on image elements (pixels) or groups of image elements (pixel groups) Is also possible. As a result, a correlation is introduced to obtain a spatiotemporal signature. In general, any variation of signal evaluation that allows dimensional numbers or data for comparison between signal characteristics and the theoretical principle of heat propagation in solids can be applied. For example, spatio-temporal spectral line changes, sequence recordings, region fragments, any desired pixel configuration or pixel pattern can be used. The important thing is to consider or include the spatial and temporal aspects together, or without it, it is almost impossible to make a reliable description of the probability of defects.

  This test method can be used in a test apparatus in which both the test object and the recording apparatus for recording the thermographic image are stationary. This means that the same measurement area in the thermograph image recorded continuously in time corresponds to the same image area (same image coordinates) in the thermograph image, so that the temperature profiles can be assigned to each other appropriately in position. Considerably simplifies the process.

  However, in a preferred embodiment, the test method is used to test elongate test objects such as, for example, steel bars, tubes, wires or the like. To test an elongated test object, a relative movement parallel to the direction of movement between the test object and a recording device for recording a thermographic image, which extends conveniently parallel to the longitudinal direction of the elongated test object. Is generated. In this case, the recording device is stationary and the test object is preferably moved relative to the recording device. The relative motion is generated such that the surface regions recorded by the thermographic images continuously recorded in time are arranged offset by a specific distance parallel to the moving direction. In this case, it is preferred that the surface areas recorded directly directly in time partially overlap so that each position of the surface under test is recorded by two or more thermographic images. As a result, it is possible to test the entire surface of an elongated test object moving in the longitudinal direction. Each position on the surface of the specimen exists in 3 or more thermographic images, for example 4 to 20 or more thermographic images, and this position is It is preferable to be at a different point (image position) in each.

  Positionally proper assignment of temperature profiles of different thermographic images presents particular problems in moving test subject tests. In a variation of the method, a first thermographic image recorded at a first time point of a series of thermographic images is analyzed by image processing and thermographic data of first surface details having anomalies that are likely to be defects. Identifying at least a first selected image detail containing. The same surface details are then automatically found in the second image details corresponding to the first image details. The second image details are located in a second thermograph image recorded at a second time after a certain time interval from the first thermograph image. Next, an integrated evaluation of the thermographic data of the first and second image details is performed to achieve a positionally appropriate assignment.

  A surface containing a defect-like anomaly in the second thermographic image to identify a path whose surface details cover the direction of travel between the first time point and the second time point for automatic search The expected position of the details is based on the measured or some other known relative velocity between the test object and the recording device and the time interval elapsed between the first time point and the first time point Are preferably specified. As a result, the evaluation of the second thermograph image can be performed from the beginning on the surface details where abnormalities that are likely to be defects were found in the analysis of the first thermograph image recorded before that time. Become.

  In order to find defect-like anomalies, it is desirable to look for a local maximum of temperature values in at least one linear or planar temperature profile in the first thermographic image. A suitable image processing filter routine can be used for this purpose.

The invention also relates to a thermographic test apparatus set up to carry out the method for spatial resolution detection and identification of defects near the surface to be tested. This test equipment
A heating device for heating the part under test so as to create a thermal imbalance between the defect area affected by the defect and the material of the test specimen free of defects;
At least one recording device for recording a series of at least two thermographic images consecutive at a time interval;
An evaluation device for evaluating the thermograph data of the thermograph image;
Including
The evaluation device is for identifying a time-dependent change of the temperature value from the temperature profile for a number of measurement positions of the measurement region recorded by the temperature profile and based on at least one evaluation criterion characterizing the heat flow in the measurement region In order to evaluate the change over time, it is configured to identify a position profile that is properly assigned from the thermograph image.

  The recording device is preferably a thermal radiation sensitive area scan camera having a number of image sequences whose image information is evaluated together.

  The above and further features will become apparent not only from the claims but also from the description and drawings, wherein each individual feature is in each case an embodiment of the invention and other fields. Or implemented as a plurality of sub-coupled forms to constitute an advantageous and inherently protectable embodiment. Exemplary embodiments are shown in the drawings and are described in more detail below.

1 shows an embodiment of a test device for a thermographic test of an elongated test object of a conductive material by a flow-through method. An example of the temperature profile recorded perpendicularly | vertically with respect to the moving direction of a test object is shown. FIG. 4 shows a schematic plan view of a heated part of a moving test object within a recording area of a thermal camera, with selected image details shown in an enlarged state, including defects. FIG. 5 is a description of a method for a positionally correct integrated evaluation of temperature profiles recorded at different times on the same area of the surface. Fig. 5 shows the positionally properly assigned details of the temperature profile in the region of the fault not caused by a crack, in the part of the temperature profile in the region where the local temperature is maximum. Figure 5 shows the positionally properly assigned details of the corresponding temperature profile in the cracked region near the surface of the portion of the temperature profile in the region where the local temperature is maximum. Fig. 4 shows the change over time of characteristic variables for non-cracking faults that characterize heat flow in the region of maximum local temperature. Figure 3 shows the change over time of a characteristic variable for a crack near the surface that characterizes heat flow in the region of maximum local temperature. Details from a temperature profile with local temperature maximum due to reflection are shown. FIG. 6B shows the temporal development of local change in temperature in the region of maximum local temperature shown in FIG. 6A. Figure 2 shows the development over time of two characteristic variables that characterize heat flow in the local maximum region.

  FIG. 1 shows a schematic diagram of an embodiment of a thermographic test apparatus 100 for testing the entire surface of an elongated test object of conductive material by a flow-through method. In the case of this example, the test object 180 is a steel billet having a rectangular cross section, which comes from a rolling device (not shown), for example using a transport device (not shown) such as a roller conveyor. In the moving direction 184 (arrow) extending parallel to the longitudinal axis 182, the sheet is conveyed at a substantially constant flow velocity Vp from a range of 0.1 m / sec to 1.5 m / sec. After hot rolling, the steel billet is not a bright surface but has a so-called “black skin” surface, whose surface temperature is typically between 0 ° C. and 50 ° C. Thus, the evaluation of the thermograph test and the thermograph data recorded thereby is explained by examining the surface 185 of the specimen at the microscopic level. Corresponding tests are performed simultaneously on the other three surfaces to be tested.

  The test apparatus has an induction heating device 110 for heating the part of the test object that enters the effective area of the heating device, and there is a thermal failure between the defect area affected by the defect and the material under test without the defect. Allow equilibrium to occur. The heating device includes an induction coil 112 designed as a flat flow-through coil for the test object with a coil plane aligned perpendicular to the flow-through direction. The induction coil is electrically connected to an alternating voltage generator 115, which is connected to the central controller 130 of the test apparatus for its operation. When the induction coil 112 is excited with an alternating voltage of an appropriate frequency, eddy currents are induced in the region near the surface of the test object, and when the test object flows through the induction coil, the region near the surface is raised above the ambient temperature. Heat. The heating process is usually relatively uniform in areas free of surface defects. However, if microstructural disturbances such as cracks, cracks, voids or the like occur in the induced current cross section, they act as electrical resistances and deflect the current. This results in a higher current density and results in power loss in current confinement. Although locally limited, the area affected by defects directly adjacent to the microstructural fault is hotter than the surrounding area without the fault, so power loss in this microstructural fault Revealed by additional occurrences. Thus, local heating is seen for lower temperature levels in the more remote surrounding areas. The typical temperature difference between the cracked area and the unaffected peripheral area material immediately adjacent to it is often on the order of magnitude from about 1K to 10K. These local temperature increases and their spatiotemporal manifestations are used in test methods for local resolution detection and identification of defects near the surface.

  In the case of this example, the generator has a maximum power output of 150 kW and an AC voltage frequency in the range of 10 kHz to 350 kHz is used. A heating device having other specifications can be used in the same manner. For example, an AC voltage generator can be operated with a power output with a maximum output of several MW, which is advantageous, for example, for test objects of larger dimensions (for example with a diameter of greater than 800 mm). The frequency range can be adapted to the measurement task. For example, eddy current penetration depth decreases with increasing frequency, resulting in a decrease in measurement, so frequencies up to 1 MHz can be useful in finding particularly small defects near the surface. Higher frequencies are also advantageous when testing conductive steels with high electrical resistance and magnetic penetration close to 1 because they quickly and locally heat the defect area relative to the surrounding area.

  The heating device puts the entire system including the test object / defect into a thermal imbalance. Using the test method and the test apparatus, it is possible to observe the situation where the system resists the state of thermal equilibrium in both the position domain and the time domain.

  For this purpose, the test apparatus has a local resolution recording device 120 sensitive to thermal radiation, which records a two-dimensional thermographic image with a high frequency image of up to 100 images per second (100 frames per second). A recording device, also referred to below as a “heat-sensing camera”, is connected to the central controller 130 to control image recording and to take over and evaluate the thermograph data acquired in the thermograph image. Integrated in this controller is a computer-based image processing system that is set up for the purpose of evaluating thermographic data identified from thermographic images based on different criteria. Such a heat-sensing camera provides a visual representation of the presence of microstructural disturbances and some of their properties based on local temperature values or based on locally identified thermal radiation, and in an associated evaluation system These abnormalities can be automatically evaluated using any suitable image processing means.

  The thermal sensing camera 120 is an area scan camera, also referred to herein as an image field 122, which in this case is a rectangular recording area that covers the entire width of the test specimen surface 185 beyond the outer edge. 122. In this example, the thermal camera 120 covers an image field 122 with a resolution of 640 × 512 pixels (image elements) and a size of 270 mm × 216 mm. The image elements (pixels) in this case correspond to relatively small rectangular surface details of 0.5 to 0.8 mm diameter on the surface 185 of the specimen. The thermographic image recorded by the area scan camera extends substantially parallel to the longitudinal direction (ie in the y direction) with many lines extending substantially perpendicular to the longitudinal direction (y direction) of the test object. It consists of columns. The thermographic image is evaluated on a column-by-column basis, particularly in order to reliably detect longitudinal flaws. A narrow measuring area 124 of straight nature associated with a row of heat sensitive cameras extends transverse to the defect 188. This measurement region is often called a measurement line.

At the time t ₁ shown in FIG. 1, the defect 188 near the surface is in the form of a longitudinal crack, and more or less in the longitudinal direction of the test object in the vicinity of the entry side of the recording area facing the induction coil 112. It extends in parallel. The position of the same longitudinal crack at the time t ₂ > t ₁ and t ₃ > t ₂ thereafter is indicated by a dotted line, and the exact same defect or the exact same surface details are recorded in the heat sensitive camera at different times. Although the image position in the thermograph image can be arranged in the region 122, the image position of the thermograph image recorded at a specific distance in the moving direction 184 depending on the flow-through velocity V _p and continuously generated in a certain time interval. It is shown that the respective time intervals are offset with respect to each other in the direction of travel 184.

  The image recording frequency used for a thermal sensing camera is a number of thermostats, such as at least 5, or at least 10, or at least 15 where each surface portion of the surface 185 of the specimen is recorded at time intervals from one another. The flow rate of the test object is adapted to appear at different positions in the graph image.

  The display and operation unit 140 connected to the control device has a screen that can display data and relationships identified from the thermograph image. Using a keyboard and / or other input means, the test apparatus can be conveniently set up for various test tasks and operated by an operator.

In addition, a speed measuring device 150 for _obtaining the motion speed V _p to be tested at a predetermined time is also connected to the control device 130. In this example, the device acts as a position encoder and operates in a non-contact manner using laser irradiation. In other embodiments, for example, a contact position encoder with wheels that roll on the surface of the specimen can be provided.

  The accuracy of the thermographic test method can be greatly affected by variations in the degree of radiation on the surface of the specimen recorded by the thermograph. In order to minimize the negative impact on the result as much as possible, the measurement surface of the test piece is used, for example, by uniformly moistening the surface of the test piece with a liquid such as water using a humidifier 160 before passing through the induction coil. Active homogenization of the degree of radiation is performed. This technique has been found to be effective at surface temperatures of up to 50 ° C. primarily to avoid the occurrence of pseudo-displays due to local variations in the degree of radiation.

  If the abnormality is clearly identified as a defect by the test device, the automatic marking device 170 connected to the control device 130 can be used to mark the defect by spraying dyes, etc. Possible rework of a surface or separation of a part that is intentionally severely damaged is possible.

  In the following, a preferred variant of the test method that can be implemented using a test device for local resolution detection and identification of defects near the surface of the test object flowing through the test device at a high flow rate will be described. The area close to the surface to be tested is inductively heated by the induction coil 112, and the local maximum temperature occurs in the area of cracks and other microstructure defects. After the corresponding part of the test object has passed the induction coil, these areas are cooled again. The recording device 120 is provided immediately after the induction coil in the direction of motion and records the surface area during this cooling phase.

  In the first method step, thermal anomalies in the portion of the surface of the test specimen moved into the recording area 122 are identified. For this purpose, the corresponding column assigned to the incoming side can be evaluated, for example to obtain a local resolution temperature profile (line profile) perpendicular to the flow-through direction along the measurement line 124. FIG. 2 shows such a temperature profile as an example. The position POS of the measurement location in a linear measurement region extending in the x direction, perpendicular to the direction of movement (y direction), indicates the number of corresponding pixels (image elements) in the image field column. Shown above. The y-axis shows the amplitude AMP of thermal radiation assigned to the location, and in this example the absolute surface temperature in degrees Celsius. It is clear that the surface temperature between the outer edges (generally pixel numbers 90 and 540) is in the range between 55 ° C. and 60 ° C. and varies locally by a few K. The temperature profile includes two anomalies, approximately a first local temperature maximum ST at pixel number 150 and a second local temperature maximum DEF at approximately pixel number 495. At both local temperature maxima, the temperature difference ΔT relative to the immediate surrounding region is approximately 6 to 7K. The evaluation will be described in more detail later, but the first local temperature maximum ST is due to interference not caused by cracks or other fine structure disturbances, and the second local temperature maximum DEF is actually due to cracks near the surface. Indicates that it has occurred. It is clear that the magnitude of the temperature difference ΔT is not a reliable criterion for distinguishing between actual microstructural faults and other abnormalities that are not caused by microstructural faults.

  Each thermographic image includes a number of such temperature profiles that resolve locally in the x direction. The occurrence of the local temperature maximum is automatically recorded by the image processing evaluation software, and the value of the pixel or group of pixels in the temperature profile is compared with the temperature value of the adjacent pixel or group of pixels using a suitable filter routine. Based on this comparison, such local temperature maxima can be clearly identified and distinguished from other artifacts such as abrupt temperature drop at the edges, for example. In filtering, the evaluation software operates row by row in a strip that extends transverse to the direction of motion, each containing a number of adjacent temperature profiles. FIG. 3 shows such a strip 125 that includes a defect 188. In this evaluation, the probability of the presence of crack-like defects in the longitudinal direction is increased when a significant height local temperature maximum occurs at approximately the same pixel point in a number of adjacent temperature profiles within the strip.

  The test method is not only based on the evaluation of the spatial temperature profile, ie these temperature profiles representing the local temperature distribution, but also on the analysis of their changes over time. This combination is also referred to herein as spatial-temporal analysis. For this purpose, it is not sufficient to analyze a single temperature profile; instead, multiple temperature profiles recorded at certain time intervals relative to one another are positioned relative to one another on the same measurement area of the surface. Appropriately set, the dynamic spatiotemporal behavior of the development of temperature distribution can be analyzed.

  In the test method embodiments described herein, a special variation of pattern detection was used to identify defects in a previous time thermograph image that could represent a defect in a later recorded thermograph image. The anomalies are repositioned properly in position, thereby providing the possibility of acquiring multiple temperature profile time series from the same measurement area, regardless of the movement of the test object relative to the heat sensitive camera. For this purpose, strips 125 associated with specific surface details of the first thermographic image at an early stage are evaluated row by column and analyzed for the presence of anomalies, in particular local temperature maxima. Based on the individual column temperature data, adjacent regions are calculated that contain the region of maximum local temperature that calibrates the anomaly. The selected rectangular image details 128 containing the defect 188 are shown in the left strip 125 and the right magnified view of FIG. The local coordinates of the selected image details 128, i.e. the position in the thermograph image, indicates the position of the relevant surface portion of the test object containing the defect 188 at the time of recording the first thermograph image. Image information contained within selected image details, including spatially adjacent pixels, can be treated as binary large objects (BLOBs) in image processing software and later recorded thermographic images Can be rearranged within.

  The first thermograph for the calculation of the found pattern by searching for the same pattern in the thermograph image after being recorded at intervals based on the “pattern” represented by the data structure in the area around the defect 188 Find the image details that correspond as closely as possible to the surface details used in the image analysis. Preferably, image details corresponding to specific surface details are looked up in at least 5 to 10 consecutively recorded thermographic images and then the image information is evaluated together.

In order to spatially limit the area covered by the search in the later recorded thermographic image, thereby speeding up the evaluation, the expected surface details containing defects likely to be defective in the later recorded thermographic image. Are determined based on the movement speed V _p of the test object, the direction of movement 184, and the time interval elapsed between the individual recording times of the thermographic image, measured using the velocity measurement system 150, each In this case, the respective distances covered by the surface details of interest between the respective first analysis and the recording times of the respective thermographic images obtained later are respectively calculated from this. Even if the through-flow velocity varies slightly, the surface part of interest or the data associated with this part can thus be rediscovered with an accuracy in the range of the measurement accuracy of the position encoder (here for example about ± 1 mm). This is known and corresponds to a positioning accuracy of the order of about ± 2 pixels on the surface of the specimen in this example. The software then makes a final correction for positionally correct overlay by tracking, i.e. by pattern recognition, by calculation, so that about ± 1 pixel or ± 0.5 mm on the surface of the specimen. Effective position accuracy can be achieved.

  This approach actually takes into account the fact that the test conditions are usually not ideal. For example, sliding between the test material and the transport system may result in bending of the test material and / or a decrease in the speed of the test material when wound on a roll, and subsequent speed increases resulting in speed fluctuations. In other cases, it leads to position inaccuracy. The resulting problems with the test are avoided by a combination of speed measurement, search for surface portions that may be affected by defects based on this measurement, and subsequent search for surface patterns (tracking).

Then, in each of the image details recorded sequentially in time, one or more temperature profiles extending beyond the location of the potential defect can be identified and evaluated together. As shown in FIG. 3B, each temperature profile location is located at the same point in the selected image portion, and each temperature profile that is properly assigned is the same linear shape on the surface under test. This measurement area extends beyond the position of a potential defect. To illustrate this, on the left side of FIG. 3B, the three image details 128, 128 ′, associated with the same surface details and recorded at different times t ₁ , t ₂ > t ₁ and t ₃ > t ₂ and A temperature profile is shown in each of the image details, which is shown as 128 "and extends beyond the defect in the x direction. In the part of the figure on the right, the temperature profiles recorded in succession in time are shown together. , The x-axis shows the position POS (x) in the x-direction, and the y-axis shows the temperature T. Thus, on a moving test object, the spatio-temporal heat propagation in the area of potential defects is very accurately Can be identified.

  Each of the temperature profiles represents a region extending transverse to the defect, in which the defect is generally centered. Each of the temperature profiles has a local temperature maximum, and its level relative to the surrounding area decreases with time (eg quantified by the temperature difference ΔT), while in the location domain, for example half of the maximum However, the maximum width of the local temperature given by the maximum width increases with time. Thus, with these locally properly assigned local temperature profiles recorded continuously in time, it is possible to draw quantitative conclusions about spatio-temporal heat propagation in the area of potential defects, It can be evaluated as follows.

  FIG. 4 shows together a number of positionally assigned temperature profiles together in each of FIGS. 4A and 4B, the temperature profile shown at the top of the figure being the temperature profile shown below it respectively. It was recorded at an earlier time. FIG. 4A shows a typical temperature profile for fault ST, which results in a local temperature maximum at approximately pixel number 7, but not due to a crack near the surface. FIG. 4B shows, for comparison, a positionally properly assigned temperature profile from the area of the crack-like defect DEF, again where the local temperature maximum is in the area of pixel number 7. The positionally assigned temperature profile is then analyzed based on the evaluation criteria, and the spatio-temporal expression of the temperature profile causes the spatio-temporal expression of the temperature distribution to be in the region of cracks or other microstructural defects. It is possible to draw relatively reliable conclusions as to whether the expected dynamic behavior caused by heat flow is accommodated or whether other laws are followed.

  One of the evaluation criteria or characteristic variables is the amplitude AMPM of the temperature value at the local temperature maximum position in the temperature profile. Another feature variable that has proven to be very reliable for evaluating the dynamic behavior of heat propagation is the heat concentration value KONZ in the local maximum region of the temperature value in the temperature profile. FIG. 5 shows in FIG. 5A the amplitude AMPM for a defect ST not caused by a crack and the change over time of the concentration value KONZ at various time increments t, in FIG. 5B in the same time window for a crack DEF near the surface. The change over time of the same characteristic variable is shown. The temperature difference ΔT in the peripheral region at the local maximum position with respect to the peripheral region is shown on the y-axis.

  In many tests, in the cracked area, both the rate of cooling, i.e. the temperature change at the location where the local temperature is maximum and the loss of concentration, are both relatively large and are not affected by cracks or other microstructure damage. It has been found that the corresponding values that can be shown in the region are very different. It has been found that the temperature maximum represented by the amplitude AMPM, ie the temperature at the local maximum position, decreases continuously after completion of the heating phase, ie during the cooling process, and the rate of decrease is relatively large. In the case of this example, it is assumed that the probability that a crack exists is high if the cooling rate in the region of at least five thermograph images recorded in succession is greater than a predetermined threshold for the cooling rate. The hot volume concentration value KONZ is a measure of the ratio of the amount of heat at the maximum local temperature directly compared to the adjacent surrounding area. If the heat concentration value decreases with time, this is an indication, inter alia, that heat is flowing laterally towards the surrounding area. This is the case, for example, with cracks and is therefore considered to be a sign that the observed signal was caused by heat propagation in the solid near the crack.

  On the other hand, in the case of a failure not caused by a crack, which is explained based on FIG. 5A, the heat volume concentration KONZ is lower than that in the case of a crack. It rises at the beginning and then gradually falls. The maximum amplitude AMPM also first increases and then decreases at a relatively low cooling rate, which is clearly lower than the expected cooling rate in the cracked region (FIG. 5B).

  Other variations of the spatiotemporal behavior of the heat volume concentration may result from the typical behavior caused by heat flow in the presence of defects, and can be used as an indication of failure not due to cracks or the like. For example, the hot volume concentration value may remain generally constant over a relatively long period of time, or appears to rise or fall disproportionately.

  These examples make it possible to reliably distinguish the different causes of the local temperature maximum first found in the temperature profile by analysis and quantitative evaluation of the spatiotemporal expression of the temperature profile. For the first anomaly found, if the features described in connection with FIGS. 4B and 5B apply in principle, the cause is classified as a crack and, if appropriate, the corresponding surface portion is marked by the marking device 170. The On the other hand, if the spatiotemporal analysis shows an atypical behavior of cracks, voids and other microstructure defects (see, eg, FIGS. 4A and 5A), no cracks are shown. In this way, pseudo display can be avoided with high reliability. Inclusion of spatiotemporal heat propagation in the area of potential defects contributes decisively to interference suppression in detection and defect identification using thermographic signals.

Based on FIG. 6, by way of example, it will be described how spatio-temporal heat propagation analysis contributes to interference suppression. For this purpose, FIG. 6A shows the details of the temperature profile, which contains a very clear local temperature maximum with a temperature difference ΔT of at least 10 K relative to the surrounding area, for example in the region of the pixel 455. To do. For some conventional test systems, such an indication is automatically considered an obvious sign of the presence of deep cracks, and the test object is marked and possibly discarded accordingly. However, a spatio-temporal analysis of heat propagation shows that there is no risk of cracking. In FIG. 6B, a temperature profile is shown that is properly allocated from the local maximum region for different time points. A special feature compared to the profile of FIG. 4 is that the profile with the largest amplitude is at a later time point (t ₂ > t ₁ ) than the profile with a clearly smaller amplitude recorded at the previous time point t ₁ . It was recorded. As shown in FIG. 6C, the abnormality can also be seen from the temporal change of the characteristic variable of the local maximum amplitude (AMPM) and the heat volume concentration value (KONZ). Both values increase with time, but this cannot be explained by the heat propagation in the locally heated cracked region. For this example, the strong local temperature maximum shown in FIG. 6A may be due to reflections at corresponding locations on the surface of the specimen. Since the development of the temperature profile over time does not show the typical propagation behavior of the cracks in any way, such reflections are therefore not classified as cracks. On the other hand, reflections are likely to be misinterpreted as cracks in some conventional systems.

  Other feature variables can also be used as evaluation criteria in lieu of or in addition to the feature variables described herein as examples. Derivatives of the described time function, such as changes in cooling rate over time, can also be used for this purpose. Since heat propagation can essentially be explained by the solution of the heat diffusion equation, fitting the Gaussian curve or error function can quantify the temperature profile over time in the local maximum region, in these cases Well fitting makes it possible to assume that there is heat propagation dominated by heat flow, but if not well, other causes are suggested. Further, by applying a polynomial as an approximate function to the temperature profile and analyzing the polynomial coefficient, it is possible to distinguish a defect (for example, a crack) that is being searched for from an interference (for example, a reflection) that is not important.

100: Thermograph test device 110: Induction heating device 112: Induction coil 115: Alternator 120: Local resolution recording device, thermal sensing camera 122: Recording region 124: Measurement line 125: Strips 128, 128 ′, 128 ″: Image details 130: Control device 140: Display and operation unit 150: Speed measurement device 160: Humidification device 170: Marking device 182: Longitudinal axis 184: Movement direction 185: Test piece surface 188: Defect

Claims (8)

A thermographic test method for local resolution detection and identification of defects near the surface to be tested, comprising:
Create a thermal imbalance between the defect area affected by the defect and the material under test that is free of defects so that the peripheral area without defects in the defect area is not heated or heated more strongly than the defect area. Heating the part to be tested;
A lateral heat flow, each representing a local temperature distribution in the surface area of the test object recorded by a thermograph image, flows laterally from the locally heated defect area to the peripheral area of the defect area. Recording a series of thermographic images that are continuous at time intervals within a heat propagation phase that begins when
Identifying from the thermograph image a positionally properly assigned temperature profile that is a local resolution profile;
Different positions in the temperature profile are each assigned a value of a measured variable representing the temperature at the respective position, and each positionally properly assigned temperature profile is the position of the test object. Assigned to the same measurement area of the surface,
Obtaining a time-dependent change in temperature value from the temperature profile for a number of measurement points in the measurement region recorded by the temperature profile;
Evaluating the change over time based on at least one evaluation criterion characterizing the transverse heat flow in the measurement region;
Including
Searching for at least one local maximum of the temperature values in the temperature profile in the evaluation;
In the evaluation, determine the thermal volume concentration values in a local maximum of the region of the temperature values in the temperature profile, to evaluate the time course of thermal volume concentration value,
The heat volume concentration values were compared with the adjacent peripheral region, thermographic test wherein a measure der Rukoto ratio of heat in the direct local temperature maximum. 2. The evaluation according to claim 1, wherein in the evaluation, in addition to evaluating the change with time of the heat volume concentration value, the change with time of the amplitude of the temperature value in the region having the maximum local density is evaluated. Thermographic test method as described.   Thermometer according to any one of the preceding claims, characterized in that in the evaluation, at least 3, preferably from 4 to 20, positionally properly assigned temperature profiles are evaluated together. Graph test method.   In order to test an elongate test object, a relative movement is generated between the test object and a recording device for recording the thermographic image in a direction of movement that preferably extends parallel to the longitudinal direction of the test object. The surface area recorded by the thermograph image is offset with respect to the direction of motion, and the surface area of the thermograph image recorded directly continuously preferably overlaps in an overlapping area, The thermograph test method according to any one of claims 1 to 3.   The thermograph test method according to claim 4, wherein the recording device is provided in a stationary form, and the elongated test object is moved with respect to the recording device. Analyzing a first thermographic image recorded at a first time point from a series of thermographic images for identification of at least a first found image detail containing surface details having defect-like anomalies Steps,
In the second thermograph image recorded at a second time point after a certain time interval from the first thermograph image, the second image details corresponding to the first image details are automatically obtained. Find step and
Including
Said first image details and details the second image, in order to identify the likely defect anomaly, characterized in that are found in Rukoto to explore the local maximum of the temperature values in the temperature profile, wherein The thermograph test method according to any one of claims 4 to 5.   For automatic search, the expected position of the surface details containing the defect-like anomaly in the second thermographic image is the relative speed between the test object and the recording device, the direction of the movement, And, based on the time elapsed between the first time point and the second time point, preferably the relative speed, in particular the speed of the test object, is measured. The thermograph test method according to claim 4, claim 5 or claim 6. A thermographic test device for local resolution detection and identification of defects near the surface to be tested,
Create a thermal imbalance between the defect area affected by the defect and the material under test that is free of defects so that the peripheral area without defects in the defect area is not heated or heated more strongly than the defect area. A heating device (120) for heating a portion of the test object (180);
At least one recording device (120) for recording a series of at least two thermographic images continuous at a time interval;
An evaluation device for evaluating the thermograph data of the thermograph image;
Including
A thermographic test apparatus, wherein the test apparatus is configured to perform the method according to any of claims 1-7.

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